Ceramics refer to non-metal inorganic solid material prepared by thermal treatment at a high temperature, and are superior in resistance to fire, acidity and alkalinity. Substrate for ceramic material, abrasive material, filling material and pearlescent pigments should have a flaky shape, uniform size and thickness, smooth surface and relatively low agglomeration between particles.
Improvements of thermal conductivity and mechanical strength are necessary for being used as ceramic material and filling material. For this purpose, thin thickness and uniform particle size are required, and agglomeration should be lowered for homogeneous dispersion and distribution.
Important properties in pearlescent pigments as a substrate are particle size, shape, surface property and a refractive index. That is, uniform particle size is necessary for achieving clear colors because the ratio of reflected light to transmitted light are different between large-sized particles and small-sized particles. Further, the particle size is closely related to the wavelength of light, and greatly affects the tinting strength of pearlescent pigments.
Surface area increases with the decrease of the particle size, thereby increasing tinting strength and reflectivity and thus producing clear colors. However, it is difficult to uniformly coat small-sized particles with a metal or metal oxide, and the small size may decrease the effect of light interference and deteriorate the pearl gloss. Therefore, particles are preferred to have sufficiently uniform size for expression of various pearl gray colors.
The substrate in pearlescent pigments should also be transparent particles that have a uniform thickness and a smooth surface because these properties affect the expression of pearl gray color such as tinting strength and hiding power. When the thickness of particles is not uniform or surface is not smooth, most light reflects or scatters on the surface. Agglomeration of particles and non-uniform thickness also inhibit the expression of various pearl gray colors when the particles are coated with a metal or metal oxide.
Therefore, for being useful in pearlescent pigments, flaky alpha-alumina crystals should be transparent flaky particles that have an average particle thickness of 0.5 μm or less; an average particle size of 15 μm or higher, while having very smooth surface without causing agglomeration.
Examples of the conventional substrate of pearlescent pigments include lead carbonate, bismuth oxychloride (BiOCl) and natural or synthetic mica. The mica is also used as a reinforcing agent for improving the ductility and mechanical property of ceramic material or as an additive for improving thermal conductivity.
Meanwhile, the flaky alumina substrate has been conventionally prepared by using a hydrothermal method or by using titanium dioxide or tin dioxide as additives for being used as a substrate of pearlescent pigments.
The hydrothermal method has problems of small particle size and low large aspect ratio. The method of titanium dioxide addition comprises no aging process and thus it is difficult to obtain flaky alpha-alumina crystals with uniform particle size and superior dispersity despite superior properties in a substrate of pearlescent pigments. The tin dioxide method results in relatively high thickness despite large size.
Therefore, aluminum solution is hydrolyzed into a pseudo-boehmite normally according to solution chemistry method. The pseudo-boehmite undergoes phase transition into gamma-alumina (γ-Al2O3) at above 400° C. Hexagonal flaky crystals may be formed by changing the gamma-alumina (γ-Al2O3) into alpha-alumina in molten salt solution by means of heat treatment to 1,200° C.
The flaky alpha-alumina crystals should have thin and uniform plane, a large aspect ratio (=diameter/thickness) of 50 or higher along with transparency so that it may show pearlescent gloss effect of various colors when coated with a metal or metal oxide.
However, the conventional flaky crystals have a relatively low large aspect ratio due to large thickness, and thus fail to show superior properties as a substrate of pearlescent pigments.
The present inventors have filed a patent application directed to flaky alpha-alumina crystals comprising aluminum oxide and zinc oxide as an essential ingredient and a method of their preparation as a result of search on flaky alpha-alumina crystals with large aspect ratio [Korean Patent Application No. 2005-25126]. Thus prepared flaky alpha-alumina crystals have a thickness of 0.1-0.5 gm, a diameter of 15-25 μm and a large aspect ratio of 50-250.
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Therefore, the present inventors have exerted extensive researches to overcome the morphological problems of flaky alumina crystals, widely used as a substrate of the conventional pearlescent pigments, and particularly to improve the aspect ratio of the flaky alpha-alumina crystals disclosed in Korean patent application No. 2005-25126 filed by the present inventors. As a result, they found that flaky alpha-alumina crystals that comprise aluminum oxide, zinc oxide and tin oxide as an essential ingredient at a predetermined weight ratio may have an average particle thickness of 0.5 μm or less; an average particle diameter of 30 μm or higher and particularly an aspect ratio of 100 or higher. They also found that the flaky alpha-alumina crystals may be prepared by preparing a mixed gel by means of hydrolysis of an aqueous aluminum precursor solution containing aqueous flux, aqueous zinc precursor solution and tin precursor aqueous solution, and conducting aging, drying and crystallization processes under predetermined conditions, thereby finally completing the present invention.
Accordingly, the present invention aims to provide flaky alpha-alumina crystals comprising aluminum oxide, zinc oxide and tin oxide as an essential ingredient, and their preparation method.
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